Conventional random access memories store only one bit of information per unit memory cell. For static random access memory (SRAM), a flip-flop gate comprising a pair of transistors stores the bit of information. For dynamic random access memory (DRAM), a capacitor in the memory cell stores a charge that is indicative of the bit of information.
The bit of information is typically stored in the unit memory cell by selecting and assigning two voltage levels each corresponding to either a logic "1" or a logic "0". For SRAMs, the flip-flop gate stores a logic "1" as the condition of one transistor being on and the other transistor being off and stores a logic "0" as the condition of the transistors being reversed.
For DRAMs, the capacitor stores the charge in a first voltage range (typically a higher voltage range), such as 3-5 volts, to indicate a logic "1" and in a second voltage range (typically a lower voltage range), such as 0-2 volts, to indicate a logic "0".
As disclosed in the U.S. Patent application Ser. No. 08/136,161 filed Oct. 15, 1993, the subject matter of which is incorporated herein by reference, a single DRAM memory cell stores multiple bit logic signals by dividing a voltage range into discrete voltage levels. For example, a memory cell may store four bits by dividing a 4 volt range into sixteen 0.25 volt ranges: bits 0000 are stored as a voltage between 0 and 0.25 volts; bits 0001 are stored as a voltage between 0.25 and 0.5 volts; and so forth bits 1111 are stored as a voltage between 3.75 and 4.0 volts. In general, the width of the voltage level equals the voltage range divided by 2 to the power of the number of bits.
The decay time for the capacitors in the memory cells is not uniform at different voltage levels. Hence, the refresh time of the memory cell is not equal for different logic levels. In a memory system using such memory cells, the refresh time of the system is selected based on the logic level having the fastest decay time. Thus, this refresh time limits the performance of the system.